Technical Abstract:
Studies of the genetic structure of the Iowa Corn Borer Synthetic #1 (CB) and Iowa Stiff Stalk Synthetic (SS) maize populations are of particular significance because these are the heter otic groups that form the basis of the modern hybrid corn industry. Both populations have undergone reciprocal recurrent selection (RRS) since 1949 and have had complementary alleles fixed through selection. Previous studies had analyzed 82 RFLP loci in CB and SS at two time points (Cycle 0 and Cycle 12). These results led us to believe that examining changes in allele frequencies in CB and SS is a valuable approach to exploring genome-wide changes resulting from RRS. We saw evidence for changes in allele frequency at 17% of all loci measured that could not be explained by genetic drift. Instead, these changes appear to be due to selection and, therefore, warranted further investigation. We measured the progress of this program in terms of between- and within-population genetic differentiation by analyzing the variation at 86 SSR loci among plants sampled from eight groups (progenitors, Cycle 0, Cycle 1, Cycle 3, Cycle 6, Cycle 9, Cycle 12, and Cycle 15) in each population. The progenitors used to form these populations show a high amount of variation (3.38 alleles/locus). As the RRS program proceeds, the variation decreases (Cycle 15, 1.88 alleles/locus). Plants within groups have a larger amount of genetic variation (66%) than groups within populations (13%) or between populations (21%). The repartitioning variation from within populations (96% in progenitors) to between populations (58% in Cycle 15) over time is consistent with theoretical expectations of divergence between populations. These intermediate time points represent a comprehensive genetic look in CB and SS to evaluate the molecular changes occurring as a result of RRS.